The present invention relates to a process for the recovery of gold and silver from used refractory bricks. More particularly, the present invention relates to a process for the recovery of gold and silver from the used refractory bricks of Dore bricks of Dore furnace. The invention also relates to a process for the recovery of gold and silver from the used refractory bricks generated during the processing of the anode slimes of copper industry, through a non-cyanide route. The invention will be useful for gold producing, gold refining and copper producing industries.
Usually when copper metal is produced from its sulphide ores such as chalcopyrite by pyrometallurgical route, precious metals like gold and silver enter into the copper metal. During electrorefining of copper, these precious metals get collected into the anode slimes. These slimes are then made free from the associated selenium, tellurium and copper etc. and smelted in the Dore furnace to get the Dore Bullion containing gold and silver. During the smelting of the anode slimes, a portion of the gold and silver enter into the refractory bricks of the Dore furnace. At the end of their life cycle, the used refractory bricks are replaced with new bricks. Such used refractory bricks are also generated in gold production plants. The amount of gold and silver entering into the refractory bricks is substantial. A survey of published literature and patents does not disclose any solution to address the problem of recovering gold and silver from the used refractory bricks of Dore furnace generated in the processing of the anode slimes of copper industry, through a non-cyanide route.
The main object of the present invention is to provide a process for the recovery of gold and silver from the used refractory bricks of the Dore furnace, which obviates the drawbacks as detailed above.
Another object of the present invention is to provide a process for the recovery of gold and silver from the used refractory bricks of the Dore furnace generated during the processing of the anode of copper industry through a non-cyanide route.
Accordingly, the present invention provides a process for the recovery of gold and silver from the used refractory brick of the Due furnace which comprises.
(i) grinding used refractory bricks of the Dore furnace generated during the processing of the anode slimes of copper industry or from gold production plants to 80% to 200 mesh size
(ii) roasting the ground refractory brick powder along with sodium chloride as an additive at a temperature in the range of 400-900xc2x0 C.
(iii) leaching the said roasted powder with ammoniacal sodium thiosulphate solution at a temperature in the range of 40-90xc2x0 C. for 4-10 hrs and separating the solids and liquid by filtration
(iv) precipitating silver from the leached liquor obtained from step (iii) as silver sulphide and converting it to metallic silver by heating at a temperature in the range 600-800xc2x0 C.
(v) leaching the solid residue obtained from step (iii with ammonium sulphate solution at a temperature in the range of 40-90xc2x0 C. for 4-10 hrs to remove the acid consuming gangue constituents and filtering the solution
(vi) leaching the solid residue obtaining from (v) with acidic thiourea solution in presence of ferric sulphate at a pH of 1 to 2.5 in the temperature range of 20-50xc2x0 C. for 2-6 hrs and filtering
(vii) mixing activated charcoal with the filtered leach liquor obtained from step (vi) and agitating for 1-2 hrs for adsorbing the gold onto the activated charcoal
(viii) filtering the activated charcoal loaded with gold and incinerating it at a temperature of 700-800xc2x0 C., for 2-3 hrs
(ix) smelting the incinerated charcoal along with the metallic silver obtained from step (iv) to get gold-silver bullion and
(x) parting gold and silver from the gold-silver bullion.
In an embodiment of the invention the ratio of the Dore furnace brick powder to the leachant used is in the range of 1:3 to 1:10.
In another embodiment of the invention, the sodium thiosulphate used for the leaching of silver has a concentration ranging between 5 g/l to 70 g/l.
In another embodiment of the invention the thiourea solution used for leaching the gold has a concentration ranging between 0.05 M to 2.0 M.
In yet another embodiment of the invention, the activated charcoal employed is selected from wood charcoal and coconut charcoal and used is in an amount in the range of 0.25 g/l to 10 g/l of the thiourea leach liquor.
In yet another embodiment of the invention the amount of ammonium sulphate employed to leach out the gangue constituents is in the range of 20 g/l to 100 g/l.
In still another embodiment of the invention, the amount of ferric sulphate used along with thiourea during the leaching in the range of 0.5 g/l to 10 g/l.
The process of present invention involves in solubilizing the silver and gold from the used Dore furnace bricks with the help of two non-cyanide lixiviants such as sodium thiosulphate and thiourea respectively. The reactions involved in the recovery are given below:
2 Ag+4S2O3 2xe2x88x92+H2O+1/2 O2 =2 Ag(S2O3)23xe2x88x92+2OHxe2x88x92
Au+2CS (NH2)2 +Fe3+=Au [CS(NH2)2]2++Fe2+
The silver is precipitated from the thosulphate leached solution as silver sulphide and then converted to metallic silver by heating. The gold dissolved in the thiourea solution is adsorbed onto activated charcoal. The loaded carbon after incineration is smelted along with the metallized silver obtained from thiosulphate leaching to get gold-silver bullion. From the mixed bullion, gold and silver are separated by parting.
The process gives a recovery of 94% of gold and 96% of silver from the used refractory bricks of the Dore furnace.
The novelty of the present invention is recovering gold and silver from the used refractory bricks of the Dore furnace with the help of environmentally safe reagents.